Wireless communications represent an enabling technology for modern culture. While there are seemingly endless varieties of wireless communication devices such as cellular telephones, two-way pagers, and wireless personal digital assistants, all such devices incorporate essentially common functionality. Information, voice or otherwise, is transmitted between a given wireless device and a remote user or system through a supporting wireless communications network. With the increasing popularity of wireless communication devices, designers are forced to devise methods of supporting ever-increasing numbers of device users in a finite bandwidth.
Most contemporary schemes for supporting a large number of communication device users within a given bandwidth are based on digital transmission techniques. Unlike conventional analog communications systems, such as the North American Advanced Mobile Phone System standard (AMPS), the newer digital transmission standards involve both envelope and phase (or frequency) modulation techniques, and require precise transmit power control. Whereas a communications device transmitting under the AMPS standard typically used a power amplifier biased for saturated or quasi-saturated operation, digital transmission standards impose strict requirements for transmitted signal fidelity and transmitted signal power, as well as strict limitations on adjacent channel power, which is a measure of interference between adjacent radio channels. These strict standards mandate the use of linear or quasi-linear power amplifiers. Appropriate amplifier bias networks are critical in achieving acceptable amplifier performance.
Power control is an essential element in most digital transmission schemes. Oftentimes, the transmitted signal power must vary linearly over a range of as much as 35 dB. One method of achieving transmit signal power control involves varying the amplitude of the radio frequency signal to be amplified by the power amplifier, while configuring the power amplifier to have a fixed gain. Thus, an associated bias network must provide the correct amount of amplifier bias current over widely ranging input and output signal magnitudes. This type of bias network must typically support both small and large signal operation of the associated power amplifier. Ideally, the bias circuit provides bias current proportional to the input signal--the radio frequency signal to be amplified--power or amplitude over the expected input signal range. Obviously, at the highest levels of input signal power, significant bias current magnitude may be required from the bias network in order for the power amplifier to linearly amplify the input signal.
For some types of power amplifiers, the power amplifier process technology has intrinsic characteristics that can reduce the amount of bias current required from the bias network with increasing input signal power. Power amplifiers implemented using gallium arsenide (GaAs) heterojunction bipolar transistor (HBT) technology exhibit increasing gain with increasing emitter current density. Essentially, HBT amplifiers require proportionately less bias current at higher levels of output power. Other types of process technologies do not exhibit similar characteristics. For example, silicon germanium (SiGe) is a promising process technology in that it allows the integration of logic gates and power transistors, while exhibiting good high frequency characteristics. However, bipolar transistors implemented in SiGe tend to exhibit gain compression, in which their gain tends to fall or at least flatten beyond a certain level of output signal power. Indium phosphide (InP) represents another otherwise promising power amplifier process technology exhibiting similar problems with gain compression.
Ideally, the bias network would provide adequate bias current to insure linear or quasi-linear operation across the full range of operating power for the power amplifier. Because of gain compression, however, the magnitude of bias current required at higher power levels is significant. Existing approaches to linear amplifier bias network design are not adapted to provide such significant levels of bias current. Because of the radio frequency signals involved, and because of cost and design considerations, power amplifier bias networks for use in portable communication devices should involve as few components as possible, and should adopt relatively straightforward circuit architectures.
Accordingly, there remains a need for an economical linear power amplifier bias network that embodies the desirable characteristics of low component count and good radio frequency signal response, while being able to provide the significant levels of bias current necessary for certain types of transistor power amplifiers. Ideally, such a bias network would be configurable so that it could be made compatible with a wide range of power amplifier types, having a broad range of bias current requirements. In order to satisfy the need for significant bias currents at high levels of output power, the needed bias circuit architecture must be configurable to have a bias current gain that may be adjusted to greater than unity if needed in a particular application.